Field of the Invention
The present invention relates to a method for producing a Group III nitride semiconductor single crystal and to a method for producing a GaN substrate. More particularly, the invention relates to a method for producing a Group III nitride semiconductor single crystal and to a method for producing a GaN substrate, which methods employ a flux method.
Background Art
A variety of methods for producing a semiconductor crystal are known, and examples thereof include vapor phase growth methods such as metalorganic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), and liquid phase epitaxy (LPE). One technique of LPE is a flux method employing an Na flux. In the Na flux method, a molten mixture of Na (sodium) and Ga (gallium) is reacted with nitrogen at about 800° C. and some tens of atmospheres, for the growth of a GaN crystal.
In the Na flux method, a seed crystal is generally employed. Examples of the seed crystal employed in the method include a GaN substrate and a so-called template substrate, composed of a sapphire substrate and a GaN layer formed thereon through HVPE or a similar technique. Patent Document 1 discloses use, as a seed crystal, of a template substrate composed of a base substrate and an underlayer film formed thereon. The base substrate of the template substrate is made of sapphire or a similar material, and the underlayer film is formed of GaN, AlN, AlGaN, GaN/AlN, etc.
Patent Document 2 discloses an Na flux method which includes adding C (carbon) to a molten mixture. Through addition of carbon, generation of miscellaneous crystals is prevented, and nitrogen solubility is enhanced. However, mechanisms thereof have not been elucidated in detail.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2006-131454    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2011-132110
The Na flux method has a drawback in that a GaN seed crystal is molten (i.e., undergoes melting back) during the period from a start of crystal growth to the time when the nitrogen concentration of the molten mixture reaches a super-saturation level. When melting back occurs, the temperature distribution profile and composition of the molten mixture vary, and the surface of the seed crystal fails to have uniformity in thickness. Particularly in the case of addition of carbon to the molten mixture, the GaN seed crystal more readily undergoes melting back, and etching proceeds locally, thereby considerably impairing surface flatness, which is problematic. When a template substrate is employed as a seed crystal, in some cases, the template substrate partially undergoes melting back, and a part of the sapphire substrate surface is exposed. On the exposed area, GaN cannot grow.
In order to avoid the influence of melting back, the thickness of the GaN layer formed on the sapphire substrate is conventionally adjusted to as thick as 5 to 30 μm. However, forming such a thick GaN layer requires a long period of time, thereby impairing template substrate productivity. It is true that formation of a thick GaN layer avoids a problem that a GaN-non-growing area is provided due to exposure of the sapphire substrate through melting back. However, the seed crystal surface fails to have uniformity in thickness. Thus, uniform crystal growth of GaN cannot be attained.
Patent Document 1 discloses that, in order to suppress melting back during crystal growth of GaN, the operation temperature is maintained at a level lower than the growth temperature, and then the temperature is elevated to the growth temperature. However, when the growth temperature is lowered, undesired miscellaneous crystals are formed. Patent Document 1 also discloses that melting back occurs not only in the case of GaN but also in the case of AlN. Therefore, melting back might possibly occur also in the case of AlGaN.
However, in contrast to the conceivable melting back of AlGaN estimated from Patent Document 1, the present inventors have found that no substantial melting back occurs in the case of AlGaN, and that the amount of melting back of the seed crystal is suppressed to 500 nm or less. The inventors have further found that the quality of a crystal formed on a seed crystal can be remarkably improved by reducing the amount of melting back of the seed crystal to 500 nm or less.
Meanwhile, in the case where a GaN single crystal is grown on an underlayer through a flux method, the crystal properties of the GaN single crystal are inherited from those of the underlayer. That is, the dislocation density of the single crystal to be formed is inherited from that of the underlayer. This feature is the same in the case of the growth disclosed in Patent Document 1. In this case, the dislocation density of the GaN single crystal is about 1×106/cm2, and a smaller dislocation density is preferred. For example, a dislocation density of 1×105/cm2 or less is preferred. Thus, in order to produce a GaN single crystal having a smaller dislocation density, the dislocation density must be considerably reduced during the growth of a GaN single crystal.
Meanwhile, when a GaN single crystal is grown through a flux method, the underlayer undergoes melting back. Generally, the surface of the underlayer which has undergone melting back is not flat and has irregularities. In the subsequent growth of a semiconductor single crystal, some dislocations are bent, and, as a result, dislocations extending from the irregularities decrease. Although melting back can reduce a part of dislocations, the effect of reduction is not sufficient. Since melting back occurs in a nonuniform manner, difficulty is encountered in reduction of dislocations in the entire wafer.